Brassinosteroids Alleviate Flooding-Induced Physiological Decline in Shade-Acclimated Tree Tomato Plants by Enhancing Osmoprotectant Accumulation, Photosynthetic Performance, and Shoot Growth

The tree tomato (Solanum betaceum Cav.) is susceptible to waterlogging stress, which under natural conditions often coincides with low radiation. Brassinosteroids (BRs) have emerged as a tool to mitigate waterlogging stress. This research evaluated the effect of exogenous BRs on growth, gas exchange, chlorophyll a fluorescence, photosynthetic pigments, proline, and soluble sugars in shade-acclimated tree tomato plants under semi-controlled waterlogging (WL). Shading pre-acclimation simulated the low-radiation environment that accompanies flooding in the tropical Andes. Nine-month-old plants were subjected to waterlogging for three days in 60 L bag pots, with a BR applied (0, 1.5, and 3 µM) before and midway through the stress. Two controls were included: shaded plants without WL (S) and an absolute control (CK). Because no WL treatment was evaluated under full-sun conditions, the individual effects of waterlogging and shading cannot be fully disentangled, and their interaction could not be assessed. Furthermore, the absence of a shade + BR treatment without WL limits the ability to distinguish BR-specific protective effects from general BR-induced physiological enhancement. Therefore, the study primarily characterizes BR-mediated responses to waterlogging in shade-acclimated plants. The WL (0 μM BR) treatment reduced chlorophyll, CO2 assimilation, leaf water potential, photosystem II (PSII) electron transport, and leaf and root growth, with greater damage during reoxygenation. BR treatment, particularly at 3 µM, increased osmoprotectants and photosynthetic pigments, enhancing quantum efficiency, water use efficiency (WUEe), net photosynthesis, and membrane integrity, supporting physiological recovery after reoxygenation and reducing shoot-growth inhibition, though roots showed no significant improvement. Foliar BRs may be a promising agronomic tool to mitigate waterlogging damage in shade-acclimated tree tomato, especially under climate-change scenarios of increasing flooding.